Plasma deposition is a technique that is widely used in industry to deposit thin films of material on a variety of workpieces. For example, in the manufacture of integrated circuits, an RF-excited plasma comprising an inert gas, such as argon, and the reactive species silane, (SiH.sub.4,) and ammonia, (NH.sub.3,) is used to deposit a thin film of SiN, the designation given to compositions approaching Si.sub.3 N.sub.4 but not necessarily identical with Si.sub.3 N.sub.4, on a silicon wafer approximately 3 inches in diameter. The SiN film can then be used as mechanical protection during subsequent processing of the wafer, as a dielectric insulating layer between 2 layers of metallization, or (if improved) as a hermetic seal.
It will be appreciated that among the desirable properties that must be possessed by such a silicon nitride film are uniformity of thickness, uniformity of composition and freedom from defects such as impurities and pin-holes. Unfortunately, the prior art plasma deposition processes are unpredictable and such undesirable defects are quite common, occurring randomly, for no apparent reason.
One mechanism that has been proposed to explain at least some of the observed defects is a reaction of the form: EQU (SiH.sub.n *).sub.A +(NH.sub.m *).sub.B .fwdarw.(Si.sub.x N.sub.y H.sub.z).sub.C +other fragments
where
4 4 and m.ltoreq.3, PA1 where * indicates some metastable excited configuration and where the component Si.sub.x N.sub.y H.sub.z can be in an excited state or in a ground state, the said reaction taking place in the plasma rather than on the surface of the substrate.
As might be expected, such a reaction would produce particulate matter that would deposit onto the surface of the substrate. One reason that has been suggested for the occurrence of this reaction is that, once a molecule of Si.sub.x N.sub.y H.sub.z has formed in the plama, it provides all of the incentives for plasma deposition that the substrate itself provides.